The present invention relates to secondary batteries and, in particular, to second batteries having a large capacity and improved characteristics for rapid charging and rapid discharging.
In accordance with current widespread use of various small sized cordless apparatus, the demand for batteries as a power source is rapidly increasing. Particularly, from the point of view of convenience in use, a demand for batteries having a large capacity, which make it possible to use an apparatus for a long operating time with one charge of the batteries, has increased.
The demand for increased capacity in batteries used by consumers has been strong, and accordingly, development in technology for increasing the capacity of batteries has been continued for a long time. As for current batteries of this type, there are a nickel-metal hydride battery and a lithium secondary battery. A negative electrode composed of a hydrogen storage alloy as a main component is used in the nickel-metal hydride battery. The nickel-metal hydride battery has approximately the same characteristics in battery voltage, discharge behavior, and other factors, as a nickel-cadmium battery, providing interchangeability with the nickel-cadmium battery, and is noted as a battery which can be expected to increase the battery capacity by 50-100%. Further, the lithium secondary battery is a battery with a large capacity similar to the nickel-metal hydride battery and is popular because of its high battery voltage and light weight. Accordingly, the nickel-metal hydride battery and the lithium secondary battery are expected to be used in most types the cordless apparatus in the near future. Furthermore, in consideration of future depletion of oil reserves, environmental problems such as destruction of the ozone layer by carbon dioxide, and a flattening of the power demand, use of the above described batteries will become important as large scale power sources for electric cars and dispersed type power applications storage.
In view of the convenience of use of the battery, an improvement in the rapid charging characteristics of the battery, which indicates the extent to which the battery can be charged in a short time, has been the subject of increased demand. On the other hand, in consideration of an apparatus which requires a large discharge current, such as an electric car, a rapid discharge characteristic is also important. If the above described characteristics are insufficient for a desired application, usage of the battery for that purpose is restricted extremely. A lead battery and nickel-cadmium battery have sufficient characteristics with certain limits both in their rapid charging behavior and their rapid discharging behavior, but the nickel-metal hydride battery and the lithium secondary battery actually do not have sufficient characteristics with respect to rapid charging or rapid discharging behavior.
Hitherto, various methods to improve the nickel-metal hydride battery as to its rapid charge-discharge characteristics have been disclosed. For instance, there has been proposed use of an electrode made of a hydrogen storage alloy composed of ultrafine particles having an average particle size equal to or less than 5 microns (JP-A-60-119079 (1985)), use of a sheet-shaped electrode made of a hydrogen storage alloy containing a binder provided with pores at least 30 microns in diameter (JP-A-61-153947 (1986)), and use of an electrode made of hydrogen storage alloy particles (mother particles), the surface of which is coated with particles of a metal, a nickel base alloy, or stainless steel having an average diameter of 1/10.about.1/200 (JP-A-64-6366 (1989)). Regarding a lithium battery, a method for improving the rapid charge-discharge characteristics by coating the surface of a current collector has been disclosed (JP-A-5-159781 (1993)).
Generally speaking, an electrode is made of a porous plate which is prepared by pulverizing a material effective for a battery reaction to fine powder, and subsequently, forming the fine powder into a sheet by adding an adhesive thereby, or by binding the particles the fine powder by sintering. Accordingly, decreasing average diameter of the particles produces a more effective as porous material layer for battery reaction to the extent of the decrease in diameter of the particles, and also produces an increase in the field area of the reaction. However, practically, the finer the pulverized material relating to the battery reaction is, the more will be the powder fall out from the electrode. Therefore, there are problems with such measures, such as lowering of the capacity of the battery and the forming a coating film composed of impurities at the surface of the material relating to the battery reaction in the pulverizing process, with the result that the coating film causes a resistance against the battery reaction which makes the rapid charge-discharge characteristics worse.
Forming pores at the surface of the material relating to the battery reaction is effective in increasing the field area for the battery reaction, but the forming of pores at surface of the adhesive or a boundary of the particles is futile. Providing the electrode with a plurality of pores will cause the electrode to have a decreased packing density of the material relating to the battery reaction, and practically, the capacity of the battery will be decreased. In accordance with a method wherein conductive particles are arranged around a particle of the material relating to the battery reaction, various shapes of the conductive particles such as a fiber or film, can be used. And, various kind of the conductive particles, such as carbon, a metal, and a reaction catalyst, can be used so long as it may does not affect the conductivity. However, the addition of such a material which does not have any effect on the battery reaction, or a material which has only a very slight effect, will cause a problem to decrease a volumetric density.
There are various methods of coating the current collector, but their sole object is to decrease the contact resistance between the current collector and the material relating to the battery reaction. However, practically, the resistances inside the electrode, such as a contact resistance between the particle, and a reaction resistance at a boundary between a particle and the electrolyte, are significantly larger than the contact resistance between the current collector and the material relating to the battery reaction.
As described above, no effective method for improving the rapid charge-discharge characteristics has been disclosed yet.